Hybrid casting-hot working process for shaping magnesium, aluminum, zinc and other die casting metals

ABSTRACT

Molten metal is thickened by addition of high surface area aerogel powder to produce a thixotropic mass which is workable at low working forces and retains its worked shape after removal of the applied working forces. Cooling to below melting temperatures solidifies the mass into a product having the metallurgicalstructural characteristics of a forged metal notwithstanding the use of casting methods in fabrication.

United States Patent Allen 1 Mar. 21, 1972 [54] HYBRID CASTING-HOT WORKING References Cited PROCESS FOR SHAPING MAGNESIUM, UNITED STATES PATENTS ALUMINUM, ZINC AND OTHER DIE 3 286 334 11/1966 CASTING METALS Inventor: Lloyd R. Allen, 480 Pleasant Street,

Belmont, Mass. 02178 Filed: Sept. 14, 1970 Appl. No.: 72,214

U.S. Cl ..l64/55, 164/76, 72/253,

72/377 Int. Cl ..B22d 27/20 Field of Search ..164/55, 76, 97; 72/253, 377

Hay ..l64/97X 3,523,569 8/1970 Quass..."

Primary Examiner-J. Spencer Overholser Assistant Examiner-John E. Roethel Attorney-Oliver W. Hayes and Jerry Cohen [57] ABSTRACT Molten metal is thickened by addition of high surface area aerogel powder to produce a thixotropic mass which is workable at low working forcesand retains its worked shape after removal of the applied working forces. Cooling to below melting temperatures solidifies the mass into a product having the metallurgical-structural characteristics of a forged metal notwithstanding the use of casting methods in fabrication.

10 Claims, No Drawings HYBRID CASTING-HOT WORKING PROCESS FOR SHAPING MAGNESIUM, ALUMINUM, ZINC AND OTHER DIE CASTING METALS The present'application relates to the working of the socalled die casting metals-i.e., the light metals, aluminum and magnesium, and also lead, zinc and copper in elemental or alloy form, including brass, etc., through modified press forging type procedures (including extrusion, hot rolling, forging, hot swaging, hot pressing, or through modified casting procedures to yield forging type resultant products). It has been discovered that improvements in such processes and/or resulting products can be made and such improvement is the subject of the present invention.

BACKGROUND Modern hot working methods for high strength-metals, particularly light metals, have reached points of diminishing returns in capital facility expansion. For instance the US. metals industry now repeatedly requests the US. Government to fund a 100,000 ton press for use in aircraft structure forming. A 50,000 ton press is already in use in the US. for forming a 6-foot by 24-foot aluminum wing spar and like products. Other presses over 10,000 are extant in the U.S.; similar press capacity has been installed in the U.S.S.R., Germany and the United Kingdom. On a smaller size level, die casting has been useful in making small metal parts of various types at some sacrifice in desired structure. Powder metallurgy has also displaced hot work methods at some sacrifice in desired structure. However some of the traditional advantages of product structure afforded by hot working (flow lines, impact strength and control of grain size) are sacrificed when casting or powder metallurgy are resorted to.

OBJECTS The objects of the present invention are to provide an improved method of hot working in the nature of a hybrid casting-hot working process to afford low force fabrication and product structure characteristics similar to worked metals despite the use of casting methods.

GENERAL DESCRIPTION A die casting metal is melted and to the melt is added a finely divided aerogel in sufficient amount to make the molten metal thixotropic. The melt is stirred to uniformly disperse the aerogel. The aerogel does not resettle when the agitation is stopped. The thixotropic melt is worked by conventional forging type methods modified by order of magnitude decrease in the forces involved-thereby allowing substantial derating of the size and strength of the press used by orders of magnitude-eg, a shaped block, having a weight (per unit of surface area contacted) in excess of flow stress of the-melt, resting (floating) on the thixotropic melt will slowly form the melt whereas normal hot forging would require high impact speed and/or high forces in a massive press to accomplish the same task, the ratio of forces involved in the two cases being one to a hundred or more.

Viewing the same concept as a modified casting process, the thixotropic melt of die casting metal can be cast in a way to yield forging type structure. For instance the melt can be die cast and, since its thixotropic character avoids turbulence, the casting (when cooled) will exhibit a forging type grain flow pattern.

With further thickening, the thixotropic melt could be spread onto a static or moving surface (e.g., table or conveyor belt or rotating drum) coated with a refractory separating agent such as boron nitride or made of such a material to yield a mill product (sheet, tube, wire, etc.) with characteristic worked structure of high force working after freezing of the melt.

Carrying this a step further, the present method may be applied to lamination or cladding. One thixotropic melt can be spread out onto a refractory surface and then another layer of thixotropic melt can be spread over the first layer before or after freezing of the first layer.

Similarly, lamination or coating of one layer of die casting metal to another layer or substrate can be made by spreading a thixotropic melt on the other layer or substrate and freezing.

In the foregoing lamination, cladding and coating uses, the amount of inter-diffusion of layers across interfaces is controlled by the speed of freezing which can be accelerated by chilling the composite through contact with water carrying coils, and other conventional cooling or refrigeration means.

The method of the invention also comprises the formation of composite structures with the die casting metal forming a matrix for a reinforcing phase which is introduced and dispressed therein along with the aerogel.

The following non-limiting examples illustrate the properties of the thixotropic melts and principles of use of the invention:

EXAMPLE 1 Pure aluminum metal was melted in a crucible. A quantity of Cab-O-Sil silica aerogel was mixed into the melt which was agitated to froth up the melt and insure good dispersion of the aerogel. The quantity of aerogel added was a weight of l percent of the weight of the metal melt. The melt became thixotropic.

The melt was frozen into an ingot, a few pieces of which were cut and rolled into 0.020 inch thick tapes for testing of properties. The tapes exhibited a tensile strength of 30,500 p.s.i. (compared to 13,000 for the original metal alloy), elastic modulus of 10.7 X 10 p.s.i. (compared to 10.3 for the original metal alloy), proportional limit of 11,100 p.s.i. and strain at failure of 0.42. Metallographically, the frozen thixotropic melt looked like a eutectic alloy.

EXAMPLE 2 Elemental zinc metal was substituted for the aluminum of Example 1 and similarly treated with the exception that the melt was frozen very slowly-a procedure which would ordinarily induce large grains (about one-half inch diameter). It was observed that grain size in the slowly frozen melt was onesixteenth inch.

EXAMPLE 3 Aluminum was mixed with aerogel to form a thixotropic melt as in Example 1 and cooled to a block form. The block was ground to a rod 1 inch long and of half-inch diameter.

A two piece plaster mold was made with a cavity in both pieces in the form of a wrench.

The rod was remelted in a furnace by heating to about 700C. The rod held its basic shape under this heating. The rod was moved to the mold and placed in the center of the mold cavity of a lower piece of the mold and the rod extended above the surface of the lower piece. The upper piece of the mold was applied and pressed down by hand. The metal filled the now completely closed cavity to form the wrench.

The mold was opened and the wrench product was cooled and removed. Macroetching (polishing an edge and dipping in 5 percent Na 0 H solution) showed an apparent flow pattern typical of forged structures.

The freezing and remelting of the thixotropic ingot of Example 3 are optional steps, only necessary for intermediate storage. The originally formed thixotropic melt can be ladled or otherwise placed in undefined form into the cavity. The volumetric amount of thixotropic metal should be equal to or only slightly in excess 1-2 percent) of the volume of the cavity to be filled.

What is claimed is:

1. Method of fabricating a die casting metal into a wrought product in a manner to produce forging type grain structure in the wrought product without the application of forging type forces, the method comprising steps of,

a. introducing a thixotropy inducing agent into a die casting metal and maintaining the metal in the molten state with the thixotropy inducing agent dispersed through the melt, the agent being present in the melt in a weight amount of at least 0.1 percent of and no greater than 5 percent of the weight of the melt,

b. forming the melt into a wrought product shape with the application of forces of less than one one-hundredth of forces required for hot working of the same die casting metal,

c. cooling the melt to form said wrought product in solid form.

2. The method of claim 1 wherein the forming step (b) is 6. The method of claim 3 wherein the forming step (b) is continuous pressing to form an elongated mill product such as sheet rolling, tub and wire drawing, extrusion and the like.

7. The method of claim 1 wherein the thixotropy inducing agent is an aerogel powder.

8. The method of claim 1 wherein the forming comprises spreading the melt over a surface to form it as a sheet layer.

9. The method of claim 8 wherein two such melt layers are spread into contact with each other to form a laminate.

10. The method of claim 1 wherein a second material of dispersed form is introduced into the melt and dispersed therein along with the thixotropy inducing agent and held therein in dispersed form as the melt cools to a solid to form a composite material with the die casting metal as a matrix reinforced by the second material and the thixotropy inducing agent. 

2. The method of claim 1 wherein the forming step (b) is casting.
 3. The method of claim 1 wherein the forming step (b) involves actual working of the thixotropic melt to form metallurgical working structure in resultant solid.
 4. The method of claim 2 wherein the forming step (b) is die casting.
 5. The method of claim 2 wherein forming step (b) is forging.
 6. The method of claim 3 wherein the forming step (b) is continuous pressing to form an elongated mill product such as sheet rolling, tub and wire drawing, extrusion and the like.
 7. The method of claim 1 wherein the thixotropy inducing agent is an aerogel powder.
 8. The method of claim 1 wherein the forming comprises spreading the melt over a surface to form it as a sheet layer.
 9. The method of claim 8 wherein two such melt layers are spread into contact with each other to form a laminate.
 10. The method of claim 1 wherein a second material of dispersed form is introduced into the melt and dispersed therein along with the thixotropy inducing agent and held therein in dispersed form as the melt cools to a solid to form a composite material with the die casting metal as a matrix reinforced by the second material and the thixotropy inducing agent. 